The structures of phenolic silanes are well known in the literature and are disclosed in U.S. Pat. No. 3,328,450. The utility of these phenolic silanes as coupling agents for polyester laminates were discussed in E. P. Plueddemann, H. A. Clark, L. E. Nelson and K. R. Hoffman, The Society of Plastics Industry, Inc., 17th Annual Meeting of the Reinforced Plastics Divison, Febuary 6-8, Section 14-A, 1 (1962). 3-(4-Hydroxy-3-methoxyphenyl)propyltrimethoxysilane was found to have excellent force transmission properties for glass fiber reinforced epoxy resins under evaluated temperatures or after exposure to boiling water by A. T. DeBenedetto, J. A. Gomez, C. L. Schilling, F. D. Osterholtz and G. Haddad, Materials Research Society Symposium Proceeding, 170, 297 (1990).
A characteristic of these phenolic silanes is that they are unstable. The phenolic hydroxyl group undergoes a transesterification reaction with the alkoxysilyl or acyloxysilyl group to yield oligomers and polymers with high viscosity that may gel. In addition, the oligomers and polymers are very difficult to disperse in water because they are hydrophobic and are not water soluble. An essential end-use requirement is that the phenolic silanes need to be dispersible in water or in aqueous organic solvents, such as mixtures of water with alcohols, ketones, esters or ethers.
The influence of silane spacer groups on the hydrolytic stability of silica reinforced poly-(2,2-bis-[4-(methacryloxy)-2-(hydroxypropyl)phenyl]propane was investigated by N. Nishiyama, K. Horie and T. Asakura, from: Interfaces in Polymer, Ceramic, and Metal Matrix Composites, H. Ishida ed., Elsevier Science Publishing Co. Inc., 279 (1988). One silane studied was 4-methacryloyloxy-3-methoxy-1-(3-trimethoxysilylpropyl)benzene. R. H. Chung and W. D. Kray disclosed a series of silylated benzoate esters as an intermediate in making ultraviolet screening agents in U.S. Pat. Nos. 4,328,346 and 4,372,835. For example, they synthesized 2-methoxy-4-(3-methyldimethoxysilylpropyl)phenyl benzoate. When this silane was irradiated with ultraviolet light, it rearranged to make 2-methoxy-4-(3-methyldimethoxysilylpropyl)-6-benzoylphenol. These ester silanes are not suitable for use as coupling agents or as additives to waterborne coatings or primers because the by-products of hydrolysis, benzoic acid or methacrylic acid, are nonvolatile. The nonvolatility of these by-products prevents them from evaporating during the drying or curing process and they remain in the composite or dried coating.
The effects of hydroxyacetophenone structure on the ruthenium catalyzed alkylation using vinylsilanes was investigated by P. W. R. Harris and P. D. Woodgate, Journal of Organometallic Chemistry, 530, 211 (1997). Two products that they made were 4-acetoxy-2-(3-triethoxysilylpropyl)acetophenone and 4-acetoxy-2,6-bis-(3-triethoxysilylpropyl)acetophenone. The acetophenone structural fragment may be undesirable because it decomposes when exposed to ultraviolet light. In addition, the acetophenone may react with other ingredients in the composite or coating.
2-(4-Acetoxyphenyl)-1-methyldiclorosilylpropane and 3-(4-acetoxyphenyl)propylmethyldichlorosilane were synthesized as intermediates in the preparation of a phenolic functional silicone fluids, as disclosed in V. A. Sergeev, V. K. Shitikov, G. U. Abbasov, M. R. Bairamov, A. A. Zhdanov, T. V. Astapova and S. M. Aliev, Zhurnal Obshchel Khimii, 52, 1846 (1982). The acetyl groups were removed in the base-catalyzed hydrolysis and condensation of the chlorosilane intermediate. Chlorosilanes are not suitable for use in waterborne coatings because they are very corrosive and react very rapidly with water to generate hydrogen chloride.
Other blocking groups have been used to prevent the transesterification reaction of the phenolic hydroxyls with the alkoxysilyl groups. Several trimethylsilyl blocked phenolic chlorosilanes, such as [1-[4-[(trimethylsilyl)oxy]-3-methoxyphenyl]propyl]methyldichlorosilane, were prepared as intermediates in the synthesis of polysilanes, as disclosed by R. Horiguchi, Y. Onishi and S. Hayase, Macromolecules, 21, 304 (1988). The trimethylsilyl group was removed by treatment of the polysilane with methanol. However, trimethyl silyl groups are unsuitable for composites, filler treatments and waterborne coating applications. When the blocked silane is added to water, the trimethyl silyl group forms trimethylsilanol, a silylating agent that will react with the inorganic surfaces. The silylation of the surface with trimethyl silyl groups would inhibit the chemical bonding of the silane coupling agent and reduce its efficacy. In addition, the trimethylsilanol can condense with itself to form hexamethyldisiloxane, a water insoluble oily material. Oily materials in the waterborne coating formulations result in poor coating uniformity and often form xe2x80x9cfish-eyesxe2x80x9d on the surface of applicators or coated substrates.
The present acyl and carbonate blocked phenolic silanes are latent phenolic functional silanes that are useful as coupling agents for mineral filled composites, surface modifiers for inorganic materials and additives for coatings. These silanes can be used to treat particulate or fibrous inorganic fillers, prime inorganic surfaces, modify the surface properties of inorganic surfaces or modify end-use properties of coatings.
The general structural formula of these silanes is:
(RIC(xe2x95x90O)O)yC6RII6xe2x88x92yxe2x88x92z[CxH2xSi(ORIII)3xe2x88x92a(RIV)a]z
where RI is H, CH3 or RVO; RII is H or RVO; RIII is alkyl, aryl, alkaryl or acyl from 1 to 8 carbon atoms; RIV is hydrogen, alkyl, aryl, or alkaryl from 1 to 8 carbon atoms; RV is a linear or branched alkyl group from 1 to 4 carbon atoms; y is an integer from 1 to 3; z is an integer from 1 to 3; x is an integer from 2 to 6 and a is an integer from 0 to 2.
The general structural formula of the acyl and carbonate blocked phenolic silanes is set forth above. Additionally, the acyl or carbonate blocking group (RIC(xe2x95x90O)xe2x80x94) needs to generate by-products (RIC(xe2x95x90O)OH or CO2 and RVOH) that evaporate readily. Therefore the by-products should have a boiling point of less than 120xc2x0 C. and preferably less than 100xc2x0 C., at atmospheric conditions. This boiling point requirement can be achieve if the by-products form azeotropes with water. For example, 1-butanol is a potential by-product if the blocking group is butyl carbonate. It forms an azeotrope with water that boils at 93xc2x0 C. The formyl blocking group is preferred because it deblocks more rapidly when the silane is added to water. The formyl group is more hydrophilic and therefore the solubility of the silane in water is increased. The formyl group also hydrolyzes faster in water. For example, the hydrolysis of 4-nitrophenyl formate is 440 times faster than the corresponding 4-nitrophenyl acetate. See E. R. Pohl, D. Wu, D. J. Hupe, Journal of the American Chemical Society, 102, 2759 (1980).
Examples of RI are hydrogen, methyl, ethoxy, butoxy, isopropoxy or propoxy. Preferred RI are hydrogen or methyl. Examples of RII are hydrogen, methyl or methoxy. Preferred RII are methoxy and hydrogen. The incorporation of RII that are methoxy increase the solubility of the silane in water. The increase in water solubility shortens the time necessary to hydrolyze the alkoxysilyl ester and remove the blocking group. These RII groups are not reactive with the resins during curing process nor do they increase the formation of undesirable color during the drying process and in-use.
Examples of RIII are methyl, ethyl, acyl, formyl, propyl, phenyl, or n-butyl. It is preferred that the RIII is methyl, formyl or acyl. The hydrolysis of the methoxysilyl, formyloxysilyl or acetoxysilyl groups are faster than the other RIII groups. The replacement of the RIII groups with hydrogen, the result of the hydrolysis, aids in dissolving the silane in water.
Examples of RIV are methyl and ethyl. xe2x80x9caxe2x80x9d is preferably 0 or 1, most preferably 0. Preferably, x is 2 or 3, and most preferably 3. Preferably, y is 1 and z is 1.
Specific silanes are 4-acetoxy-1-(2-trimethoxysilylethyl)benzene, 2-acetoxy-5-(3-trimethoxysilylpropyl)anisole, 2-methoxy-5-(3-trimethoxysilylpropyl)phenyl formate, 4-acetoxy-1-(1-triacetoxysilylpropyl)benzene, methyl (3-triethoxysilylpropyl)phenyl carbonate, 2-acetoxy-4,6-bis-(3-trimethoxysilylpropyl)anisole, 1-acetoxy-2,4,6-tris(3-trimethoxysilylpropyl)benzene, 1,2-dimethoxy-6-acetoxy-4-(3-triethoxysilylpropyl)benzene, 4-[3-(methyldiethoxysilyl)propyl]phenyl formate, and 4-[3-dimethylmethoxysilylpropyl]-2-methoxyphenyl formate.
The blocking group prevents the transesterification reaction of the phenolic hydroxyl with the alkoxysilyl group. These blocked phenolic silanes are stable when the chemical is stored. They do not polymerize to form oligomers and polymers through the formation of aryloxysilyl bonds. Generally, when the phenolic reactivity is desired in a particular utility, the silane should be deblocked.
The acyl or carbonate blocking group can be removed by hydrolysis with water to form a phenolic silane and by-products (carboxylic acid or carbon dioxide and alcohol). The acyl or carbonate blocking group also can be removed by alcoholysis to form a phenolic silane and by-products (ester or dialkylcarbonate). The acyl or carbonate blocking groups can be removed before or after the silane is used in the application. For example, an aqueous solution of the blocked phenolic silane can be prepared in which the acetoxysilyl or alkoxysilyl group is hydrolyzed in water or aqueous organic solvents to form silanols, but the protecting group has not yet been removed.
These reaction conditions are temperature in the range of 2 to 60xc2x0 C. and preferably 20 to 40xc2x0 C., pH of the water or aqueous organic solvents, preferably aqueous alcohols, in the range of pH 3 to 6 and short reaction times of about less than 10 hours for acetyl protecting groups and less than 5 minute for the formyl protecting group. Under these conditions, less than 10 percent of the protecting group will have been removed. If the pH of the solution is below pH of 3 or above pH of 6 and the concentration of the acetyl or carbonate blocked silane is above 1 weight percent, the silanols may condense to form siliconates. These siliconates are undesirable because they may become insoluble in water or aqueous organic solvents. After the acyl or carbonate blocked phenolic silane has been used, such as treating an inorganic surface, the protecting group can then be removed. The removal of the protecting group can be achieved by allowing the acyl or carbonate blocked silane to remain in contact with the water or aqueous organic solvent for periods of time longer than 50 hours for acetyl protecting group or 50 minutes for the formyl protecting group. However, the protecting group can be removed more quickly by adjusting the pH of the water or aqueous organic solvent, raising the temperature or using catalysts. Deblocking occurs faster when the solution is made more acidic than pH of 3 or more alkaline than pH of 9.
Acids that are suitable for adjusting the pH are volatile mineral acids, such as hydrochloric acid or volatile carboxylic acids, such as acetic acid or formic acids. Bases that are suitable include volatile amines, preferably teriary amines, such as trimethyl amine, pyridine, triethylamine. Primary amines are not preferred because they may react with the acyl blocked silane to form nonvolatile amides.
Raising the temperature from ambient to from about 60xc2x0 C. to refluxing solvent (100xc2x0 C. degrees for water) promotes the deblocking reaction. Catalysts also may be used. These catalysts include metal ions, such as copper (I), cobalt (II), cobalt (III), manganese (II), calcium ion, titanium (IV), tin (II) and tin (IV), chelated complexes of metal ions, such as acetyl acetonate titanate chelate or ethylacetoacetate titanate chelate, or organometallic compounds, such as dimethyl tin sulfide. Removing the carbonate protecting group is more difficult, and requires catalysts, such as a strong base.
The hydrolysis conditions can be chosen so that both the silane hydrolysis and deblocking reactions occur before use. Alcoholysis can be used to remove the protecting group without hydrolyzing the acyloxysilyl or alkoxysilyl group. The alcoholysis can be achieved by reacting the acyl or carbonate blocked phenolic silane with a volatile alcohol, preferable methanol and ethanol, under ambient conditions. However, under these reaction conditions, the deblocking reactions are very slow. Generally, large excesses of alcohol are used. The deblocking reactions can be catalyzed by acids, such as hydrogen chloride, formic acid or acetic acid, bases, such as tertiary amines, lithium alcoholates, sodium alcoholates, potassium alcoholates, titanium alcoholates, zirconium alcoholates, hafnium alcoholates, or aluminum alcoholates and metal hydroxides. The acyl or carbonate deblocking reaction also is facilitated if the by-product of the alcoholysis are removed by distillation or evaporation. The hydrolysis and alcoholysis reactions are discussed in, The Chemistry of Carboxylic Acids and Esters, S. Patai, ed., Interscience Publishers, New York (1969).
The by-products that are formed upon deblocking should be volatile at ambient conditions. They should not adhere to the surface. During the drying and curing processes, these by-products evaporate or distill away from the phenolic silane and therefore will not affect the end-use properties.
The acyl and carbonate blocked phenolic silanes and the phenolic silane that is formed after the blocking group is removed are useful in many applications, especially those where high temperature performance is required. These silanes (blocked and unblocked) are capable of bonding to inorganic substrates. These silanes (blocked and unblocked) can be used to treat fillers that are used to make composites, such as phenolic break shoe linings. Potential fillers are silicas, titanium dioxide, clays, wollastanite, sand, alumina, aluminosilicates, and glass spheres. The filler may be treated with the blocked silane and be storage stable. When use is desired, the conditions should be such that the blocking group comes off.
Coatings containing these silanes have improved adhesion for applications where adherence to glass, metal or metal oxide is required, such as coil coatings. The deblocked silanes are useful for crosslinking coating and adhesives that contain hydroxyl reactive groups, such as epoxides or isocyanates or which contain alkoxy silane functional groups.
The blocked silanes can be used neat or as an emulsion.
The synthesis of the silanes can be achieve by several different approaches. One approach is to acylate a phenol that contains an alkenyl group followed by the hydrosilation with an alkoxysilane or a chlorosilane. The chlorosilane is reacted further with an alcohol, acid or anhydride. The acylation can be achieved by using acid chlorides, esters or anhydrides. Suitable acylation agents include acetic anhydride, formic and acetic anhydride, acetyl chloride, and methyl formate. Carbonate blocked phenolic silanes are made by reacting the phenol with alkyl chloroformates, dialkyl carbonates or dialkyl pyrocarbonates. Suitable reagents include methyl chloroformate, dimethyl carbonate or diethyl pyrocarbonate. The formylation of the phenolic intermediate can be done by using the procedures described by W. Stevens and A. van Es, Recueil des Travaux Chimiques des Pays-Bas, 83, 1287,1294 (1964). The acetylation of the phenolic intermediate can be done by following the procedure described by Ward and Jenkins, Journal of Organic Chemistry, 10, 371 (1945). The formation of the carbonates from the phenolic intermediate can be done by procedures referenced in H. J. Schnell, Chemistry and Physics of Polycarbonates, Interscience Publishers, New York (1964). The hydrosilation of the resulting product is done by procedures well known in the art and discussed in Comprehensive Handbook on Hydrosilylation, B. Marciniec, ed., Pergamon Press, Oxford (1992).
The acyl or carbonate blocked phenolic silanes that have two or three silyl groups can be prepared from dialkenyl or trialkenyl phenol. Representative examples of starting phenols include 2,4-divinylphenol, 2,4,9-trivinylphenol, 2,4-bis-(3-propenyl)phenol, 2,4,6-tris-(3-propenyl)phenol, and 2-methoxy-4,6-bis-(3-propenyl)phenol. The phenolic group is blocked with the acyl or carbonate groups by the procedures discussed above. The alkenyl groups are then hydrosilated with equivalent amount of the chlorosilane or alkoxysilane, as discussed by B. Marciniec, (1992). The chlorosilane are converted to the alkoxysilane or acyloxysilanes by reactions with alcohols, acids or anhydrides, as described by C. Eaborn, Organosilicon Compounds, Butterworths Scientific Publications, London (1960).
All of these references to synthetic methods are incorporated herein by reference.